One form of suppressor for ion chromatography is described in U.S. Pat. No. 4,999,098. The suppressor includes an ion receiving or regenerant channel and a sample stream or chromatographic effluent channel separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the channels. Flow from the sample flow channel is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site-to-site transfer paths across the sample flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second channel. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant. The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
U.S. Pat. No. 5,248,426 discloses a suppressor of the general type described in U.S. Pat. No. 4,999,098 in an ion chromatography system in which the effluent from the detector is recycled to the flow channel(s) in the suppressor adjacent the sample stream flow channel.
U.S. Pat. No. 5,597,481 discloses a suppressor of the foregoing type used in sample pretreatment to reduce or suppress matrix ions in the eluent of opposite charge to the analyte ions and then to analyze the analytes in their conductive forms. Using existing suppressor devices, ion exchange interactions and hydrophobic interaction of the analyte, particularly in the eluent flow channel, affects recovery of certain analytes such as oligonucleotides and oligosaccharides.
In all of the disclosed approaches, currents higher than theoretically predicted are required for achieving quantitative suppression. Under high eluent concentration conditions, this high current translates into heat generation and high background noise.
U.S. Pat. No. 6,077,434 (the '434 patent) discloses improved suppressor current efficiency for an ion chromatography membrane suppressor. Current efficiency is disclosed to be inversely related to static capacity of the sample flow channel of the suppressor. Specifically, it teaches that a decrease in the static capacity in that channel results in an increase in current efficiency leading to maximum efficiency when the channel had no capacity, such as with a neutral screen in the channel. Current efficient suppressors have the benefits of low wattage, low level of leachates, lower noise and background and fast start up times. On the other hand, static capacity in the sample flow channel provides residual capacity for suppression that could be used, particularly when no current is applied to the suppressor. The higher static capacity is also useful during installation or startup of the device when the current to the suppressor device is turned off. This allows operation of the ion chromatograph without down time.
A benefit of high static capacity is when the suppressor is used in an intermittent mode of operation as discussed in U.S. Pat. No. 5,569,365 where the power is turned off for a set duration, e.g. during suppression and separation of analytes for detection. The key benefit of this mode is low noise. Under these conditions the static capacity is used to exchange the eluent and sample counter ions.